Catalyst blends

ABSTRACT

Provided is a catalyst for the selective reduction of NOx comprising a two molecular sieve materials having a CHA structure, wherein the first molecular sieve has a mean crystal size of about 0.01 to 1 μm and the second molecular sieve has a mean crystal size of about 1-5 μm, and wherein the first molecular sieve contains a first extra-framework metal, the second molecular sieve contains a second extra-framework metal, and wherein said first and second extra-framework metals are independently selected from the group consisting of cesium, copper, nickel, zinc, iron, tin, tungsten, molybdenum, cobalt, bismuth, titanium, zirconium, antimony, manganese, chromium, vanadium, niobium, and combinations thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/461,558 (allowed), filed Mar. 17, 2017, which claims priority of U.S.Pat. No. 9,610,571, issued on Apr. 4, 2017, which claims priority ofU.S. Pat. No. 9,126,180, issued on Sep. 8, 2015, which claims priorityof PCT Patent Application No. PCT/IB2013/000096, filed Jan. 28, 2013,which claims priority of U.S. Provisional Patent Application No.61/593,030, filed Jan. 31, 2012, the disclosures of which areincorporated herein by reference in their entirety for all purposes.

BACKGROUND 1. Field of Invention

The present invention relates to catalysts, articles, and methods fortreating combustion exhaust gas, specifically selective catalyticreduction (SCR) of NO_(x) in lean-burn combustion exhaust gas.

2. Description of Related Art

The largest portions of most combustion exhaust gases contain relativelybenign nitrogen (N₂), water vapor (H₂O), and carbon dioxide (CO₂); butthe exhaust gas also contains in relatively small part noxious and/ortoxic substances, such as carbon monoxide (CO) from incompletecombustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides(NO_(x)) from excessive combustion temperatures, and particulate matter(mostly soot). To mitigate the environmental impact of exhaust gasreleased into the atmosphere, it is desirable to eliminate or reduce theamount of these undesirable components, preferably by a process that, inturn, does not generate other noxious or toxic substances.

One of the most burdensome components to remove from a lean burn exhaustgas is NO_(x), which includes nitric oxide (NO), nitrogen dioxide (NO₂),and nitrous oxide (N₂O). The reduction of NO_(x) to N₂ in a lean burnexhaust gas, such as that created by diesel engines, is particularlyproblematic because the exhaust gas contains enough oxygen to favoroxidative reactions instead of reduction. However, NO_(x) can be reducedin a diesel exhaust gas by a process commonly known as SelectiveCatalytic Reduction (SCR). An SCR process involves the conversion ofNO_(x), in the presence of a catalyst and with the aid of a reductant,into elemental nitrogen (N₂) and water. In an SCR process, a gaseousreductant such as ammonia is added to an exhaust gas stream prior tocontacting the exhaust gas with the SCR catalyst. The reductant isabsorbed onto the catalyst and the NO_(x) reduction reaction takes placeas the gases pass through or over the catalyzed substrate. The chemicalequation for stoichiometric SCR reactions using ammonia is:

2NO+4NH₃+2O₂→3N₂+6H₂O

2NO₂+4NH₃+O₂→3N₂+6H₂O

NO+NO₂+2NH₃→2N₂+3H₂O

Known SCR catalysts include zeolites and other molecular sieves.Molecular sieves are microporous crystalline solids with well-definedstructures and generally contain silicon, aluminum and oxygen in theirframework and can also contain cations within their pores. A definingfeature of a molecular sieve is its crystalline or pseudo-crystallinestructure which is formed by molecular tetrahedral cells interconnectedin a regular and/or repeating manner to form a framework. Examples ofmolecular sieves frameworks that are known SCR catalysts includeFramework Type Codes CHA (chabazite), BEA (beta), and MOR (mordenite).Catalytic performance of these molecular sieves may be improved incertain environments by a cationic exchange process wherein a portion ofionic species existing on the surface or within the framework isreplaced by transition metal cations, such Cu²⁺. In general, a highermetal loading on such molecular sieves can result in decreaseddurability of the material, particularly when the material is exposed tohigh temperatures. Accordingly, there remains a need for more durable,high performance SCR catalysts.

SUMMARY OF THE INVENTION

Applicants have surprisingly discovered that certain blends of metalpromoted CHA molecular sieves yield better performance, such as NO_(x)conversion and hydrothermal stability, compared to the performance ofeach constituent material taken separately. The CHA molecular sieves insuch blends include aluminosilicates having a low silica-to-aluminaratio (SAR) and silicoaluminophosphates (SAPOs).

The silica-to-alumina ratio (SAR) of some CHA aluminosilicates isassociated with the molecular sieve's hydrothermal stability. For suchaluminosilicates, a higher SAR corresponds to improved hydrothermalstability. However, blends of CHA SAPOs and low-SAR CHA aluminosilicatesaccording to the present invention have surprisingly demonstratedhydrothermal stability under extreme aging conditions (e.g., 900° C.).Moreover, a synergistic effect between CHA SAPOs and low-SAR CHAaluminosilicates has been found that allows blends of these materials tounexpectedly achieve better performance compared to either materialindividually. This synergistic effect is also affected by theextra-framework metal loadings on the molecular sieves.

Accordingly, in one aspect provided is a catalyst composition comprisinga blend of an aluminosilicate molecular sieve having a CHA framework anda silicoaluminophosphate molecular sieve having a CHA framework, wherein(a) the aluminosilicate molecular sieve and the silicoaluminophosphatemolecular sieve are present an aluminosilicate:silicoaluminophosphatemole ratio of about 0.8:1.0 to about 1.2:1.0; and (b) saidaluminosilicate molecular sieve contains a first extra-framework metal,said silicoaluminophosphate molecular sieve contains a secondextra-framework metal, wherein said first and second extra-frameworkmetals are independently selected from the group consisting of cesium,copper, nickel, zinc, iron, tin, tungsten, molybdenum, cobalt, bismuth,titanium, zirconium, antimony, manganese, chromium, vanadium, niobium,and combinations thereof, but preferably copper, wherein said firstextra-framework metal is present in about 2 to about 4 weight percent,based on the weight of the aluminosilicate, and wherein the weight ratioof said first extra-framework metal and said second extra-frameworkmetal is about 0.4:1.0 to about 1.5:1.0.

According to another aspect of the invention, provided is a method fortreating NOx in a lean burn combustion exhaust gas comprising: (a)contacting an exhaust gas mixture having a lamba>1 and at least one NOxcompound with a reductant and a catalyst comprising a blend of firstmolecular sieve having a CHA framework and a second molecular sievehaving a CHA framework, wherein (i) said first molecular sieve is analuminosilicate, said second molecular sieve is asilicoaluminophosphate, and said first and second molecular sieves arepresent in a mole ratio of about 0.8:1.0 to about 1.2:1.0, respectively;and (ii) said first molecular sieve contains a first exchanged metal,said second molecular sieve contains a second exchanged metal, whereinsaid first and second exchanged metals are independently selected fromthe group consisting of cesium, copper, nickel, zinc, iron, tin,tungsten, molybdenum, cobalt, bismuth, titanium, zirconium, antimony,manganese, chromium, vanadium, niobium, and combinations thereof, andwherein the weight ratio of said first exchanged metal and said secondexchanged metal is about 0.4:1.0 to about 0.8:1.0; and (b) selectivelyreducing at least a portion of said NOx into N2 and H2O; wherein saidcontacting occurs at a temperature of about 200 to about 500° C., andoptionally wherein said contacting occurs after the catalyst has beenexposed to a temperature of at least about 800° C.

According to another aspect of the invention, provided is a catalyticarticle comprising (a) a catalyst comprising a blend of first molecularsieve having a CHA framework and a second molecular sieve having a CHAframework, wherein (i) said first molecular sieve is an aluminosilicate,said second molecular sieve is a silicoaluminophosphate, and said firstand second molecular sieves are present in a mole ratio of about 0.8:1.0to about 1.2:1.0, respectively; and (ii) said first molecular sievecontains a first exchanged metal, said second molecular sieve contains asecond exchanged metal, wherein said first and second exchanged metalsare independently selected from the cesium, copper, nickel, zinc, iron,tin, tungsten, molybdenum, cobalt, bismuth, titanium, zirconium,antimony, manganese, chromium, vanadium, niobium, and combinationsthereof, and wherein the weight ratio of said first exchanged metal andsaid second exchanged metal is about 0.4:1.0 to about 0.8:1.0, and (b) aporous filter adapted for removing particulate from a disease exhaustgas, wherein said catalyst is disposed on and/or within said porousfilter. In certain embodiments, article comprises the catalyst blendwashcoated onto a flow-through or wall-flow monolith. In certainembodiments, the article comprises an extruded wall-flow filter producedfrom an extrudate containing the catalyst blend.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram of a catalyst article according to an embodiment ofthe present invention, the filter having an inlet coated with a sootoxidation catalyst zone and an outlet coated with an SCR catalyst zone;

FIG. 2 is a diagram of a wall-flow soot filter according to anembodiment of the present invention, the filter having an inlet coatedwith a soot oxidation catalyst zone and an outlet coated with an SCRcatalyst zone;

FIG. 3 is a graph depicting comparative NOx conversation data of a freshcatalyst blend according to an embodiment of the invention and NOxconversation data of the blend's constituent materials; and

FIG. 4 is a graph depicting comparative NOx conversation data of an agedcatalyst blend according to an embodiment of the invention and NOxconversation data of the blend's constituent materials.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In certain embodiments, the invention is directed to catalysts forimproving environmental air quality, particularly for improving exhaustgas emissions generated by diesel and other lean burn engines. Exhaustgas emissions are improved, at least in part, by reducing NO_(x) and/orNH₃ slip concentrations in lean burn exhaust gas over a broadoperational temperature range. Accordingly, useful catalysts are thosethat selectively reduce NO_(x) and/or oxidize ammonia in an oxidativeenvironment (i.e., an SCR catalyst and/or AMOX catalyst).

Preferred catalysts of the present invention comprise a blend ofmetal-promoted molecular sieves having a CHA structure according to theIUPAC structural code specified by the International Zeolite Association(IZA). It will be appreciated that such molecular sieves includesynthetic crystalline or pseudo-crystalline materials that are isotypes(isomorphs) of one another via their CHA Framework. Specific CHAisotypes that are useful in the present invention include, but are notlimited to, DAF-5, LZ-218, Linde D, Linde R, Phi, SAPO-34, SAPO-44,SAPO-47, SSZ-13, SSZ-62, UiO-21, and ZK-14, with SAPO-34 and SSZ-13being most preferred.

Preferred CHA molecular sieves include aluminosilicates andsilicoaluminophosphates (SAPOs). As used herein, the term SAPO means amaterial composed of tetrahedron units of PO₂₊, AlO₂ ⁻ and SiO₂ arrangedin a three-dimensional, microporous, molecular sieve. Such SAPOs aremainly composed of silicon, aluminum, phosphorus and oxygen, but can be“metal-substituted” as described below. As used herein, the term“SAPO-34”, means silicoaluminophosphates described as SAPO-34 in U.S.Pat. No. 4,440,871 (Lok) as well as any analog thereof. As used herein,the term “SSZ-13” means aluminosilicates described in U.S. Pat. No.4,544,538 (Zones) as well as any analogs thereof. As used herein, theterm “analogs” with respect to a CHA isotype means a molecular sievehaving the same topology and essentially the same empirical formula, butare synthesized by a different process and/or have different physicalfeatures, such as different distributions of atoms within the CHAframework, different isolations of atomic elements within the molecularsieve (e.g., alumina gradient), different crystalline features, and thelike.

Useful aluminosilicates may include framework metals other thanaluminum, preferably a transition metal (also known as metal substitutedaluminosilicates). As used herein, the term “metal substituted” withrespect to a CHA aluminosilicate means the CHA framework has one or morealuminum or silicon framework atoms replaced by the substituted metal.In contrast, the term “metal exchanged” means a zeolite havingextra-framework metal ions. Examples of metals suitable for substitutioninclude copper and iron. Metal substituted silico-aluminophosphate (alsoreferred to as MeAPSO) molecular sieves likewise have a framework inwhich the substituted metal has been inserted.

The CHA crystals of the present invention are not particularly limitedby shape, however rhombohedral are preferred. In addition, preferredmolecular sieves are highly crystalline.

In a particularly preferred embodiment, the catalyst comprises a blendof at least one aluminosilicate and at least one silicoaluminophosphate.As used herein, the term “blend” means a heterogeneous, preferablyuniform, combination of two or more materials either of which could beused alone for the same or similar purpose as the blend. Preferredblends comprise an aluminosilicate molecular sieve having a CHAframework and a silicoaluminophosphate molecular sieve having a CHAframework, wherein the aluminosilicate molecular sieve and thesilicoaluminophosphate molecular sieve are present in analuminosilicate:silicoaluminophosphate mole ratio of about 0.5:1:0 toabout 1.5:1.0, more preferably from about 0.8:1.0 to about 1.2:1.0.

Preferred aluminosilicates have a mole silica-to-alumina ratio (SAR) ofless than about 35, more preferably about 10 to about 30, for exampleabout 10 to about 25, from about 14 to about 20, from about 15 to about18, and from about 20 to about 25. The SAR of aluminosilicate molecularsieves may be determined by conventional analysis. This ratio is meantto represent, as closely as possible, the ratio in the rigid atomicframework of the CHA crystal and to exclude silicon or aluminum in thebinder or in cationic or other form within the channels. Since it may bedifficult to directly measure the silica to alumina ratio of molecularsieve after it has been combined with a binder material, particularly analumina binder, these silica-to-alumina ratios are expressed in terms ofthe SAR of the molecular sieve per se, i.e., prior to the combination ofthe molecular sieve with the other catalyst components.

In preferred embodiments, the CHA aluminosilicates and/or SAPOs are inthe form of crystals having a mean crystal size of greater than about0.5 μm, preferably between about 0.5 and about 15 μm, such as about 0.5to about 5 μm, about 0.7 to about 5 μm, about 1 to about 5 μm, about 1.5to about 5.0 μm, about 1.5 to about 4.0 μm, about 2 to about 5 μm, orabout 1 μm to about 10 μm. Particularly preferred CHA aluminosilicateshave a mean crystal size of about 1 to about 3 μm. Particularlypreferred CHA silicoaluminophosphates have a mean crystal size of about2 to about 5 μm. For embodiments wherein the catalyst blend isincorporated into an extrudable composition, the mean crystal size canbe small, for example about 0.01 to about 1 μm.

Preferred SAPOs have a CHA structure and preferably have a strong:weakacid ratio and isolated acid sites.

In certain embodiments, particularly for applications involving thecatalyst blend as an extrudate, at least a portion and preferably amajority of the SAPO crystals comprise an inert surface oxide layer.Preferably, the oxide layer is an amorphous oxide disposed as a layer onthe crystal grain surface. It is understood that the coated crystalgrain comprises a body portion having a CHA framework and an oxidesurface portion, each of which have separate compositional features.Composition of the body portion can be considered to be approximatelythe same as that of a corresponding uncoated crystal grain because theoxide layer is very thin. The oxide layer mainly comprises oxides ofsilicon, aluminum and phosphorus, but may contain elements other thansilicon, aluminum and phosphorus.

The oxide layer preferably has a silicon-to-aluminum ratio (Si:Al)greater than the crystal body itself. By way of example, embodiments inwhich the SAPO crystal body has a Si:Al of about 0.05 to about 0.3, thecorresponding oxide layer on the crystal surface can have a Si:Al ofabout 0.5 or more (e.g., from about 0.5 to about 3.5). Also, the oxidelayer on the crystal grain surface preferably has an atomicsilicon-to-phosphorus ratio (Si:P) of 0.55 or more. Each of theseparameters can be measured by XPS (X-ray photoelectron spectroscopy) orwith an energy dispersive X-ray (EDX) analyzer. Other preferred SAPOcrystal grains have an Si:Al of about 0.1 to about 0.2 and acorresponding oxide layer having a Si:Al of about 0.6 to about 2 and aSi:P of about 0.6 to about 3. In general, the Si/AI and Si/P atomicratios of the surface oxide layer increase with the thickness of theamorphous oxide layer. In certain embodiments the layer has a thicknessof about 3 to about 20 nm (such as about 5 to about 15 nm, about 5 toabout 10 nm, and about 10 to about 15 nm).

The amorphous oxide layer is formed after crystallization of the SAPOcrystal grain body is almost completed. Methods for making such coatedSAPO materials can be found, for example, in EP 1512667.

The CHA aluminosilicate crystals in the catalyst composition can beindividual crystals, agglomeration of crystals, or a combination ofboth, provided that agglomeration of crystals have a mean particle sizeof less than about 15 μm, preferably less than about 10 μm, and morepreferably less than about 5 μm. The lower limit on the mean particlesize of the agglomeration is the composition's mean individualaluminosilicate crystal size.

Crystal size (also referred to herein as the crystal diameter) is thelength of one edge of a face of the crystal. For example, the morphologyof chabazite crystals is characterized by rhombohedral faces whereineach edge of the face is approximately the same length. Directmeasurement of the crystal size can be performed using microscopymethods, such as SEM and TEM. For example, measurement by SEM involvesexamining the morphology of materials at high magnifications (typically1000× to 10,000×). The SEM method can be performed by distributing arepresentative portion of molecular sieve powder on a suitable mountsuch that individual particles are reasonably evenly spread out acrossthe field of view at 1000× to 10,000× magnification. From thispopulation, a statistically significant sample of random individualcrystals (e.g., 50-200) is examined and the longest dimensions of theindividual crystals parallel to the horizontal line of the straight edgeare measured and recorded. (Particles that are clearly largepolycrystalline aggregates should not be included the measurements.)Based on these measurements, the arithmetic mean of the sample crystalsizes is calculated.

Particle size of an agglomeration of crystals can be determined in asimilar manner except that instead of measuring the edge of a face of anindividual crystal, the length of the longest side of an agglomerationis measured. Other techniques for determining mean particle size, suchas laser diffraction and scattering can also be used.

As used herein, the term “mean” with respect to crystal or particle sizeis intended to represent the arithmetic mean of a statisticallysignificant sample of the population. For example, a catalyst comprisingmolecular sieve crystals having a mean crystal size of about 0.5 toabout 5.0 μm is catalyst having a population of the molecular sievecrystals, wherein a statistically significant sample of the population(e.g., 50 crystals) would produce an arithmetic mean within the range ofabout 0.5 to about 5.0 μm.

In addition to the mean crystal size, catalyst compositions preferablyhave a majority of the crystal sizes that are greater than about 0.5 μm,preferably between about 0.5 and about 15 μm, such as about 0.5 to about5 μm, about 0.7 to about 5 μm, about 1 to about 5 μm, about 1.5 to about5.0 μm, about 1.5 to about 4.0 μm, about 2 to about 5 μm, or about 1 μmto about 10 μm. Preferably, the first and third quartile of the sampleof crystals sizes is greater than about 0.5 μm, preferably between about0.5 and about 15 μm, such as about 0.5 to about 5 μm, about 0.7 to about5 μm, about 1 to about 5 μm, about 1.5 to about 5.0 μm, about 1.5 toabout 4.0 μm, about 2 to about 5 μm, or about 1 μm to about 10 μm.

As used herein, the term “first quartile” means the value below whichone quarter of the elements are located. For example, the first quartileof a sample of forty crystal sizes is the size of the tenth crystal whenthe forty crystal sizes are arranged in order from smallest to largest.Similarly, the term “third quartile” means that value below which threequarters of the elements are located.

Large crystal CHA zeolites, such as the isotype SSZ-13, can besynthesized by known processes, such as those described in WO2010/043981 (which is incorporated herein by reference) and WO2010/074040 (which is incorporated herein by reference).

Molecular sieves in the present invention include those that have beentreated to improve hydrothermal stability. Conventional methods ofimproving hydrothermal stability include: (i) dealumination by steamingand acid extraction using an acid or complexing agent e.g.(EDTA-ethylenediaminetetracetic acid); treatment with acid and/orcomplexing agent; treatment with a gaseous stream of SiCl₄ (replaces Alin the zeolite framework with Si); and (ii) cation exchange—use ofmulti-valent cations such as lanthanum (La).

Preferably, the molecular sieves of the present invention are promotedwith one or more metals. As used herein, the term “metal promoted” withrespect to a molecular sieve means a molecular sieve loosely holding oneor more metal ions, such as copper, to the molecular sieve's frameworkas an extra-framework metal. An “extra-framework metal” is one thatresides within the molecular sieve and/or on at least a portion of themolecular sieve surface, does not include aluminum, and does not includeatoms constituting the framework of the molecular sieve.

In preferred embodiments, the catalyst comprises a CHA aluminosilicatemolecular sieve that contains a first extra-framework metal, a CHA SAPOmolecular sieve that contains a second extra-framework metal, whereinthe first and second extra-framework metals are independently selectedfrom the group consisting of cesium, copper, nickel, zinc, iron, tin,tungsten, molybdenum, cobalt, bismuth, titanium, zirconium, antimony,manganese, chromium, vanadium, niobium, and combinations thereof. Morepreferred extra-framework metals include those selected from the groupconsisting of cesium, manganese, iron, and copper, and mixtures thereof.Preferably, at least one of the extra-framework metals is copper.Preferably, both the first and second extra-framework metals are copper.Other preferred extra-framework metals include iron and/or cerium,particularly in combination with copper. The extra-framework metal canbe added to the molecular sieve via any known technique such as ionexchange, impregnation, isomorphous substitution, etc. Preferably, thefirst extra-framework metal is present in about 2 to about 4 weightpercent, based on the weight of the aluminosilicate, and wherein theweight ratio of the first extra-framework metal and the secondextra-framework metal is, respectively, about 0.4:1.0 to about 0.8:1.0.

In certain embodiments, the first extra-framework metal is present in anamount of about 1 to about 5 wt % based on the total weight of thealuminosilicate molecular sieve, for example from about 1.5 wt % toabout 4.5 wt %, from about 2 to about 4 wt %, from about 2 to about 3 wt%, or from about 2.2 to about 2.7 wt %. In certain embodiments, theextra-framework metal (M), preferably copper, is present in thealuminosilicate in an amount to produce a M:Al atomic ratio of about0.17 to about 0.24, preferably about 0.22 to about 0.24, particularlywhen the aluminosilicate has an SAR of about 15 to about 20. In certainembodiments that include exchanged copper, the copper is present in anamount from about 80 to about 120 g/ft³ of zeolite, including forexample about 86 to about 94 g/ft³, or about 92 to about 94 g/ft³.

Preferably, the second extra-framework metal is present in the SAPO inamount sufficient to achieve a weight ratio of the first extra-frameworkmetal and the second extra-framework metal of about 0.4:1.0 to about1.5:1.0, for example about 1.3:1 to about 1.1:1.0, about 0.4:1.0 toabout 0.8:1.0, and about 0.5:1.0 to about 0.7:1.0.

In one example, a metal-exchanged molecular sieve is created by blendingthe molecular sieve into a solution containing soluble precursors of thecatalytically active metal. The pH of the solution may be adjusted toinduce precipitation of the catalytically active cations onto or withinthe molecular sieve structure. For example, in a preferred embodiment amolecular sieve powder is immersed in a solution containing coppernitrate for a time sufficient to allow incorporation of thecatalytically active copper cations into the molecular sieve structureby ion exchange. Unexchanged copper ions are precipitated out. Dependingon the application, a portion of the unexchanged ions can remain in themolecular sieve material as free copper. The metal-substituted molecularsieve may then be washed, dried and calcined.

Generally, ion exchange of the catalytic metal cation into or on themolecular sieve may be carried out at room temperature or at atemperature up to about 80° C. over a period of about 1 to 24 hours at apH of about 7. The resulting catalytic molecular sieve material ispreferably dried at about 100 to 120° overnight and calcined at atemperature of at least about 550° C.

Certain embodiments of the present invention contain a relatively largeamount of Ce and have surprisingly good performance. In particular, theCHA aluminosilicate, having an SAR of less than 30, preferably about 15to about 20, that is promoted with a metal, preferably copper andpreferably in a copper:aluminum ratio of about 0.17 to about 0.24, andhas a Ce concentration of greater than 1.35 weight percent, preferably1.35 to 13.5 weight percent, based on the total weight of thealuminosilicate. Examples of preferred concentrations include aboveabout 2.5 weight percent, above about 5 weight percent, above about 8weight percent, above about 10 weight percent, about 1.35 to about 13.5weight percent, about 2.7 to about 13.5 weight percent, about 2.7 toabout 8.1 weight percent, about 2 to about 4 weight percent, about 2 toabout 9.5 weight percent, and about 5 to about 9.5 weight percent, basedon the total weight of the zeolite. For most of these ranges, theimprovement in catalyst performance correlates directly to theconcentration of Ce in the catalyst. These ranges are particularlypreferred for copper promoted aluminosilicates having an SAR of about 10to about 25, about 20 to about 25, about 15 to about 20, or about 16 toabout 18, and more preferably for such embodiments, wherein the copperis present in a copper-to-aluminum ratio of about 0.17 to about 0.24.

In certain embodiments, the cerium concentration in the catalystmaterial is about 50 to about 550 g/ft³. Other ranges of Ce include:above 100 g/ft³, above 200 g/ft³, above 300 g/ft³, above 400 g/ft³,above 500 g/ft³, from about 75 to about 350 g/ft³, from about 100 toabout 300 g/ft³, and from about 100 to about 250 g/ft³.

In certain embodiments, the concentration of Ce exceeds the theoreticalmaximum amount available for exchange on the metal-promoted zeolite.Accordingly, in some embodiments, Ce is present in more than one form,such as Ce ions, monomeric ceria, oligomeric ceria, and combinationsthereof, provided that said oligomeric ceria has a mean crystal size ofless than 5 μm, for example less than 1 μm, about 10 nm to about 1 μm,about 100 nm to about 1 μm, about 500 nm to about 1 μm, about 10 toabout 500 nm, about 100 to about 500 nm, and about 10 to about 100 nm.As used herein, the term “monomeric ceria” means CeO₂ as individualmolecules or moieties residing freely on and/or in the zeolite or weaklybonded to the zeolite. As used herein, the term “oligomeric ceria” meansnanocrystalline CeO₂ residing freely on and/or in the zeolite or weaklybonded to the zeolite.

Cerium is preferably incorporated into a CHA aluminosilicate containinga promoting metal. For example, in a preferred embodiment, a CHAaluminosilicate undergoes a copper exchange process prior to beingimpregnated by Ce. An exemplary Ce impregnation process involves addingCe nitrate to a copper promoted zeolite via a conventional incipientwetness technique.

Treatment of Exhaust Gases:

The catalyst compositions described herein can promote the reaction of areductant, such as ammonia, with nitrogen oxides to selectively formelemental nitrogen (N₂) and water (H₂O) notwithstanding the competingreaction of oxygen and ammonia. In one embodiment, the catalyst can beformulated to favor the reduction of nitrogen oxides with ammonia (i.e.,and SCR catalyst). In another embodiment, the catalyst can be formulatedto favor the oxidation of ammonia with oxygen (i.e., an ammoniaoxidation (AMOX) catalyst). In yet another embodiment, an SCR catalystand an AMOX catalyst are used in series, wherein both catalyst comprisethe catalyst blend described herein, and wherein the SCR catalyst isupstream of the AMOX catalyst. In certain embodiments, the AMOX catalystis disposed as a top layer on an oxidative under-layer, wherein theunder-layer comprises a platinum group metal (PGM) catalyst or a non-PGMcatalyst.

Preferably, the AMOX catalyst is disposed on a high surface areasupport, including but not limited to alumina. In certain embodiments,the AMOX catalyst is applied to a substrate, preferably substrates thatare designed to provide large contact surface with minimal backpressure,such as flow-through metallic or cordierite honeycombs. For example, apreferred substrate has between about 25 and about 300 cells per squareinch (CPSI) to ensure low backpressure. Achieving low backpressure isparticularly important to minimize the AMOX catalyst's effect on thelow-pressure EGR performance. The AMOX catalyst can be applied to thesubstrate as a washcoat, preferably to achieve a loading of about 0.3 to2.3 g/in³. To provide further NO_(x) conversion, the front part of thesubstrate can be coated with just SCR coating, and the rear coated withSCR and an NH₃ oxidation catalyst which can further include Pt or Pt/Pdon an alumina support. The AMOX catalyst can be used as an Ammonia SlipCatalyst (ASC) when located downstream of the SCR catalyst.

With respect to an SCR process, provided is a method for the reductionof NO_(x) compounds in an exhaust gas, which comprises contacting theexhaust gas with the catalyst composition described herein and in thepresence of a reductant for a time and temperature sufficient tocatalytically reduce NO_(x) compounds thereby lowering the concentrationof NO_(x) compounds in the exhaust gas. In one embodiment, nitrogenoxides are reduced with the reductant at a temperature of at least about100° C. In another embodiment, the nitrogen oxides are reduced with thereductant at a temperature from about 150° C. to about 750° C. In aparticular embodiment, the temperature range is from about 150 to about550° C. In another embodiment, the temperature range is from about 175to about 450° C. In yet another embodiment, the temperature range isabout 650 to about 900° C., such as about 650 to about 850° C., or about750 to about 850° C. The amount of NOx reduction is dependent upon thecontact time of the exhaust gas stream with the catalyst, and thus isdependent upon the space velocity. The contact time and space velocityis not particularly limited in the present invention. However, thepresent catalyst blends of the present invention have shown increasedNOx reduction compared to conventional SCR catalysts. As such, thecatalyst blends can perform well even at high space velocity which isdesirable in certain applications.

The reductant (also known as a reducing agent) for SCR processes broadlymeans any compound that promotes the reduction of NOx in an exhaust gas.Examples of reductants useful in the present invention include ammonia,hydrazine or any suitable ammonia precursor, such as urea ((NH₂)₂CO),ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate orammonium formate, and hydrocarbons such as diesel fuel, and the like.Particularly preferred reductant, are nitrogen based, with ammonia beingparticularly preferred. The addition of nitrogenous reductants can becontrolled so that NH₃ at the catalyst inlet is controlled to be 60% to200% of theoretical ammonia calculated at 1:1 NH₃/NO and 4:3 NH₃/NO₂. Inembodiments, the ratio of nitrogen monoxide to nitrogen dioxide in thecatalyst inlet gas is from 4:1 to 1:3 by volume. In this regard, theratio of nitrogen monoxide to nitrogen dioxide in the gas can beadjusted by oxidizing nitrogen monoxide to nitrogen dioxide using anoxidation catalyst located upstream of the catalyst.

In another embodiment, all or at least a portion of the nitrogen-basedreductant, particularly NH₃, can be supplied by a NO_(x) adsorbercatalyst (NAC), a lean NOx trap (LNT), or a NOx storage/reductioncatalyst (NSRC), disposed upstream of the dual function catalyticfilter. For example, under certain conditions, such as during theperiodically rich regeneration events, NH₃ may be generated over aNO_(x) adsorber catalyst. The SCR catalyst is capable of storing thereleased NH₃ from the NAC catalyst during rich regeneration events andutilizes the stored NH₃ to selectively reduce some or all of the NO_(x)that slips through the NAC catalyst during the normal lean operationconditions.

NAC components useful in the present invention include a catalystcombination of a basic material (such as alkali metal, alkaline earthmetal or a rare earth metal, including oxides of alkali metals, oxidesof alkaline earth metals, and combinations thereof), and a preciousmetal (such as platinum), and optionally a reduction catalyst component,such as rhodium. Specific types of basic material useful in the NACinclude cesium oxide, potassium oxide, magnesium oxide, sodium oxide,calcium oxide, strontium oxide, barium oxide, and combinations thereof.The precious metal is preferably present at about 10 to about 200 g/ft³,such as 20 to 60 g/ft³. Alternatively, the precious metal of thecatalyst is characterized by the average concentration which may be fromabout 40 to about 100 grams/ft³.

The methods of the present invention can be performed on an exhaust gasderived from a combustion process, such as from an internal combustionengine (whether mobile or stationary), a gas turbine and coal or oilfired power plants. The method may also be used to treat gas fromindustrial processes such as refining, from refinery heaters andboilers, furnaces, the chemical processing industry, coke ovens,municipal waste plants and incinerators, etc. In a particularembodiment, the method is used for treating exhaust gas from a vehicularlean burn internal combustion engine, such as a diesel engine, alean-burn gasoline engine or an engine powered by liquid petroleum gasor natural gas.

Catalytic Articles:

Typical applications using the SCR and AMOX catalysts of the presentinvention involve heterogeneous catalytic reaction systems (i.e., solidcatalyst in contact with a gas and/or liquid reactant). To improvecontact surface area, mechanical stability, and fluid flowcharacteristics, the catalysts can be supported on a substrate. Forexample, the catalyst compositions of the present invention can be inthe form of a washcoat, preferably a washcoat that is suitable forcoating a substrate such as a metal or ceramic flow through monolithsubstrate or a filtering substrate, such as a wall-flow filter orsintered metal or partial filter. Accordingly, another aspect of theinvention is a washcoat comprising a catalyst component as describedherein. In addition to the catalyst component, washcoat compositions canfurther comprise other, non-catalytic components such as carriers,binders, stabilizers, and promoters. These additional components do notnecessarily catalyze the desired reaction, but instead improve thecatalytic material's effectiveness, for example by increasing itsoperating temperature range, increasing contact surface area of thecatalyst, increasing adherence of the catalyst to a substrate, etc.Examples of such optional, non-catalytic components can includenon-doped alumina, titania, non-zeolite silica-alumina, ceria, andzirconia that are present in catalyst composition, but serve one or morenon-catalytic purposes. For embodiments in which the molecular sieve inthe catalyst contains Ce, the corresponding washcoat may furthercomprise a binder containing Ce or ceria. For such embodiments, the Cecontaining particles in the binder are significantly larger than the Cecontaining particles in the catalyst.

Washcoat composition, and particularly extrudable compositions, may alsoinclude fillers and pore formers such as crosslinked starch,non-crosslinked starch, graphite, and combinations thereof.

Preferred substrates, particular for mobile applications, includewall-flow filters, such as wall-flow ceramic monoliths, and flow throughfilters, such as metal or ceramic foam or fibrous filters. In additionto cordierite, silicon carbide, silicon nitride, ceramic, and metal,other materials that can be used for the porous substrate includealuminum nitride, silicon nitride, aluminum titanate, α-alumina, mullitee.g., acicular mullite, pollucite, a thermet such as Al₂OsZFe, Al₂O3/Nior B₄CZFe, or composites comprising segments of any two or more thereof.Preferred materials include cordierite, silicon carbide, and aluminatitanate. In a preferred embodiment, the substrate is a flow-throughmonolith comprising many channels that are separated by thin porouswalls, that run substantially parallel in an axial direction over amajority of the length of the substrate body, and that have a squarecross-section (e.g., a honeycomb monolith). The honeycomb shape providesa large catalytic surface with minimal overall size and pressure drop.

The coating process may be carried out by methods known per se,including those disclosed in EP 1 064 094, which is incorporated hereinby reference.

Catalyst compositions of the present invention are particularly suitedfor applications in which the substrate is a diesel particulate filter(DPF) or other soot filter, such as a catalyzed soot filter (CSF). Insuch embodiments, the filter substrate includes an SCR catalyst zone sothat exhaust gas passing through the filter also contacts the SCRcatalyst, thus eliminating a major portion of the NO_(x) components fromthe exhaust gas. Applying an SCR catalyst to a filter substrateadvantageously reduces the number of components & component size in theexhaust system (i.e., separate filter and SCR components are combinedinto one unit) and reduces costs. However, some conventionalhigh-performance SCR catalysts are not hydrothermally stable at the hightemperatures encountered during active regeneration of the filter. TheSCR catalysts of the present invention are hydrothermally stable totemperatures of at least 900° C., and therefore are well suited for suchapplications.

During normal operation of a diesel exhaust system equipped with a DPFor CSF, the soot and other particulates accumulate on the upstream sidesof the filter which lead to an increase in backpressure. To alleviatethis increase in backpressure, the filter substrates are continuously orperiodically regenerated (i.e., active regeneration). One form of activefilter regeneration involves intermittently introducing additionalhydrocarbon fuel into the exhaust gas and combusting that fuel to heatthe filter to a temperature required for adequate combustion of theaccumulated soot. The requisite soot combustion temperature can bereduced (for example, to about 600° C.) by coating the filter with asuitable combustion-promoting catalyst (soot oxidation catalyst).However, if during an active regeneration event, a period of low exhaustgas flow occurs, e.g. when the engine/vehicle is caused to idle, thereduced gas flow prevents heat from being removed from the CSF. This canresult in parts of the filter reaching temperatures in excess of 1000°C.

The high hydrothermal stability of the SCR catalyst of the presentinvention allows more soot to be trapped on the filter before activeregeneration is required. Less frequent active regeneration leads toimproved fuel economy. Moreover, the high hydrothermal stability of theSCR catalyst removes the need for the soot oxidation catalyst coating onthe filter. Forgoing this additional catalyst layer reduces backpressureand increases filter efficiency and fuel economy. Thus, in oneembodiment, provided is a filter substrate comprising a CSF zone and anSCR zone. In another embodiment, an SCR catalyst of the presentinvention is applied to an otherwise bare DPF, preferably in a zone onthe downstream side of the filter.

Turning to FIG. 1, shown is an embodiment of the invention comprising afilter substrate 10, such as a diesel particular filter, having an inlet14 and an outlet 15 relative to the direction of exhaust gas flow 13through the filter. The filter inlet comprises a soot oxidation zone 14,while the filter outlet comprises an SCR zone 15. As used herein, theterm “zone” means a catalytic area within and/or on the filtersubstrate. For example, a zone can be an area of the filter substrate inwhich a catalyst has permeated or a catalyst layer residing on top ofand/or within filter substrate. The zone can be a discrete area,completely separated from other zones, can be adjacent to, or overlapwith, other zones, or can be partially fused into other zones. The term“inlet” means the side, face, surface, channel, and/or portion of thefilter into which an exhaust gas typically flows from an externalsource. The term “outlet” means the side, face, surface, channel, and/orportion of the filter from which an exhaust gas typically exits thefilter. The phrase “on the inlet” and “on the outlet”, with respect tothe orientation of a catalytic zone and a filter substrate, is meant toinclude a catalyst residing as a zone or layer on top of the substrateface and/or within the substrate walls (i.e., within the pores of thesubstrate walls).

In another preferred embodiment, the substrate is a porous wall-flowfilter which is similar to the honeycomb monolith, but having channelsthat are open and capped at alternate ends in a checkerboard fashion.This way the particle laden exhaust gases are forced to flow through thewalls. Gas is able to escape through the pores in the wall material.FIG. 2 shows a wall-flow filter 20 having inlet channels 23 and outletchannels 24 which are defined by a gas permeable walls 27 and gasimpermeable inlet caps 25 and outlet caps 26. Particulates, however, aretoo large to escape and are trapped on and/or in the filter walls.Exhaust gas having a direction of flow 29 enters the filter 20 via oneor more of the inlet channels 23, passes through the gas permeable walls27 which separate the inlet and outlet channels, and then exits thefilter via the outlet channels 24. The exhaust gas entering the inletchannels typically comprises soot, NO_(x), and preferably also containsa nitrogenous reducing agent, such as NH₃, which is used to convert theNO_(x) into other gases via an SCR reaction. As the exhaust gas passesthrough the gas permeable wall, at least a portion of the particulatematter in the exhaust gas is trapped at the inlet where it contacts thesoot oxidation zone. The soot oxidation zone facilitates alow-temperature oxidation reaction wherein solid, carbonaceous particlesof the soot are converted into gases, such as CO₂ and water vapor, whichthen pass through the gas permeable filter wall. As the exhaust gaspasses through the SCR catalyst zone, at least a portion of the NO_(x)reacts with NH3 in the presence of the SCR catalyst, wherein the NO_(x)is reduced to N₂ and other gases.

The respective orientation of the soot oxidation zone and the SCR zoneare not particularly limited provided that a majority of the targetedparticulate matter contacts the soot oxidation zone in a mannersufficient for soot combustion. Thus, in certain embodiments the twozones partially or completely overlap. In other embodiments, the twozones converge between the inlet and outlet, while in other embodimentsthey are spatially separated. The zones on the inlet and outlet mayexist as a coating on the surface of the filter substrate or may diffuseor permeate into all or a portion of the filter substrate. In aparticularly preferred embodiment, the soot oxidation zone and the SCRzone permeate into opposite sides of the wall of a wall-flow filter.That is, the soot oxidation zone is created via the soot oxidationcatalyst permeating into the wall from the inlet channel side of thewall and the SCR zone is created via the SCR catalyst permeating intothe wall from the outlet channel side of the wall.

The catalyst in the CSF typically comprises at least one platinum groupmetal (PGM), but in particular embodiments it comprises Pt either aloneor in combination with one or more additional PGM, such as both Pt andPd or both Pt and RJi or all three of Pt, Pd and RJi including suitablepromoters such as Mg, Ba or rare earth metals such as Ce. The materialfrom which filter substrate monolith is made can support the catalyst orit can be supported on a surface area increasing washcoat component,e.g. particulate alumina, titania, non-zeolite silica-alumina, ceria,zirconia and mixtures, composite oxides and mixed oxides of any two ormore thereof. Typical Pt loadings range from about 50 g Pt/ft³ to about150 Pt/ft³.

The total amount of soot oxidation catalyst component in the sootoxidation zone will depend on the particular application, but couldcomprise about 0.1 to about 15 g/in³, about 1 to about 7 g/in³, about 1to about 5 g/in³, about 2 to about 4 g/in³, or about 3 to about 5 g/in³of the soot oxidation catalyst component. The total amount of SCRcatalyst component in the SCR zone will depend on the particularapplication, but could comprise about 0.1 to about 15 g/in³, about 1 toabout 7 g/in³, about 1 to about 5 g/in³, about 2 to about 4 g/in³, orabout 3 to about 5 g/in³ of the SCR catalyst. Preferred washcoat loadingfor the SCR catalyst is from about 0.1 to about 0.5 g/in³.

The actual shape and dimensions of the filter substrate, as well asproperties such as channel wall thickness, its porosity, etc., depend onthe particular application of interest. Wall flow filter useful in thepresent invention have up to about 700 channels (cells) per square inchof cross section. In one embodiment, the wall-flow filter contains about100 to 400, cells per square inch (“cpsi”).

Particular combinations of filter mean pore size, porosity, poreinterconnectivity, and washcoat loading can be combined to achieve adesirable level of particulate filtration and catalytic activity at anacceptable backpressure. In certain embodiments, the washcoat loading onthe porous substrate is >0.25 g/in³, such as >0.50 g/in³, or >0.80g/in³, e.g. 0.80 to 3.00 g/in³. In preferred embodiments, the washcoatloading is >1.00 g/in³, such as >1.2 g/in³, >1.5 g/in³, >1.7 g/in³or >2.00 g/in³ or for example 1.5 to 2.5 g/in³.

Porosity is a measure of the percentage of void space in a poroussubstrate and is related to backpressure in an exhaust system:generally, the lower the porosity, the higher the backpressure.Preferably, the porous substrate has a porosity of about 30 to about80%, for example about 40 to about 75%, about 40 to about 65%, or fromabout 50 to about 60%.

Pore interconnectivity, measured as a percentage of the substrate'stotal void volume, is the degree to which pores, void, and/or channels,are joined to form continuous paths through a porous substrate, i.e.,from the inlet face to the outlet face. In contrast to poreinterconnectivity is the sum of closed pore volume and the volume ofpores that have a conduit to only one of the surfaces of the substrate.Preferably, the porous substrate has a pore interconnectivity volume ofat least about 30%, more preferably at least about 40%.

The mean pore size of the porous substrate is also important forfiltration. Mean pore size can be determined by any acceptable means,including by mercury porosimetry. The mean pore size of the poroussubstrate should be of a high enough value to promote low backpressure,while providing an adequate efficiency by either the substrate per se,by promotion of a soot cake layer on the surface of the substrate, orcombination of both. Preferred porous substrates have a mean pore sizeof about 5 to 50 μm, for example about 10 to about 40 μm, about 20 toabout 30 μm, about 10 to about 25 μm, about 10 to about 20 μm, about 20to about 25 μm, about 10 to about 15 μm, and about 15 to about 20 μm. Inother embodiments, the mean pore size of the filter is about 10 to about200 nm.

Wall flow filters for use with the present invention preferably have anefficiency of least 70%, at least about 75%, at least about 80%, or atleast about 90%. In certain embodiments, the efficiency will be fromabout 75 to about 99%, about 75 to about 90%, about 80 to about 90%, orabout 85 to about 95%. Here, efficiency is relative to soot and othersimilarly sized particles and to particulate concentrations typicallyfound in conventional diesel exhaust gas. For example, particulates indiesel exhaust can range in size from 0.05 microns to 2.5 microns. Thus,the efficiency can be based on this range or a sub-range, such as 0.1 to0.25 microns, 0.25 to 1.25 microns, or 1.25 to 2.5 microns. Preferredporosity for cordierite filters is from about 60 to about 75%.

The soot oxidation zone and SCR zone can be incorporated into the sootfilter by any practical means. For example, the inlet channels of awall-flow soot filter can be dipped into an soot oxidation catalystcomposition at a depth and for a period of time that will allow the sootoxidation catalyst composition to permeate the filter walls to a certaindepth and/or concentration. Additional techniques, such as applicationof pressure or a vacuum, can be used to promote adequate, even, and/ormore rapid permeation of a particular coating. After the soot oxidationcatalyst composition permeates the inlet of the wall-flow filter, thefilter is dried and then the outlet channels of the filter are dippedinto an SCR catalyst composition at a depth and for a period of timethat will allow the SCR catalyst composition to permeate the filterwalls to a certain depth and/or concentration. Again, additionaltechniques, such as application of pressure or a vacuum, can be used topromote adequate, even, and/or more rapid permeation of a particularcoating. The SCR zone is then dried. One or more of the dippingprocesses can be repeated to achieve the desired coating level. Afteracceptable catalyst loadings are obtained, the catalytic coating isactivated, preferably at a temperature of about 100 to about 300° C. forabout 1 to about 3 hours. The activated filter is calcined to removeadditional moisture at a temperature of about 450 to about 550° C. forabout 1 to about 3 hours. The drying and calcining steps preferablyperformed as per standard CSF preparation conditions.

In certain embodiments, the catalyst blends of the present invention areincorporated into an extruded article, such as an extruded flow-throughfilter, instead of being coated on a substrate. Extruded filters usingthe catalyst blends of the present invention are advantageous in thatthey have lower backpressure, have more uniform catalyst distribution,are more durable, can achieve higher catalyst loadings, and/or can havethe same effectiveness with lower catalyst loading, compared to filterscoated with a catalyst washcoat. Preferred extruded catalytic filtershave about 100 to about 400 cells per inch, a wall thickness of about 7to about 15 mils, and a porosity of at least about 50%, for exampleabout 55 to about 75%, or about 60 to about 70%.

System:

Another aspect of the invention is directed to a system for treatinglean-burn exhaust gas. Such exhaust gas systems are configurations oftwo or more discrete devices or components, each of which are capable ofmodifying the composition of the exhaust gas independently of theother(s), but interact with the other(s) to form a coherent scheme fortreating the exhaust gas. Preferably, one or more of the components ofthe exhaust gas system interact to produce a synergistic result.

In a preferred embodiment, the system of the present invention comprisesa catalytic soot filter as described herein in fluid communication withan injector or other device for introducing a nitrogenous reductant intothe exhaust gas, wherein the injector or other device is disposedupstream of the filter. In certain embodiments, the system furthercomprises an exhaust gas stream generated by a lean burn internalcombustion engine, one or more conduits for carrying a flowing exhaustgas, wherein the conduits are in fluid connection with at least some ofthe components of the exhaust system, and/or a source of nitrogenousreductant.

The injector can continuously, periodically, or intermittently introducethe reductant, such gaseous ammonia, ammonia in aqueous solution,aqueous urea, or ammonia from an ammonia generator, into the exhaust gasat a dose effective for optimization of the downstream SCR reaction. Theinjector is in fluid communication with the exhaust gas stream and maybe attached, connected to, and/or integrated with a conduit, such as apipe, for directing the exhaust through at least a portion of theexhaust gas system. The injector may also be in fluid communication witha reduction agent supply tank to provide for repeated injections of thereduction agent.

In a particular embodiment, metering is controlled in response to thequantity of nitrogen oxides in the exhaust gas determined eitherdirectly (using a suitable NOx sensor) or indirectly, such as usingpre-correlated look-up tables or maps—stored in the controlmeans—correlating any one or more of the abovementioned inputsindicative of a condition of the engine with predicted NO_(x) content ofthe exhaust gas. The metering of the nitrogenous reductant can bearranged such that 60% to 200% of theoretical ammonia is present inexhaust gas entering the SCR catalyst calculated at 1:1 NH₃/NO and 4:3NH₃/NO₂. The control means can comprise a pre-programmed processor suchas an electronic control unit (ECU). Controlling the metering involveslimiting the introduction of the nitrogenous reductant into the flowingexhaust gas only when it is determined that the SCR catalyst is capableof catalyzing NO_(x) reduction at or above a desired efficiency, such asat above 100° C., above 150° C. or above 175° C. The determination bythe control means can be assisted by one or more suitable sensor inputsindicative of a condition of the engine selected from the groupconsisting of: exhaust gas temperature, catalyst bed temperature,accelerator position, mass flow of exhaust gas in the system, manifoldvacuum, ignition timing, engine speed, lambda value of the exhaust gas,the quantity of fuel injected in the engine, the position of the exhaustgas recirculation (EGR) valve and thereby the amount of EGR and boostpressure.

In certain embodiments, the system further comprises a diesel oxidationcatalyst (DOC) to oxidize hydrocarbon based soluble organic fraction(SOF) and carbon monoxide content of diesel exhaust by simple oxidation:

CO+½O₂→CO₂

[HC]+O₂→CO₂+H₂O

The DOC may also serve to oxidize NO into NO₂, which in turn, may beused to oxidize particulate matter in particulate filter. Additionally,the DOC may serve to reduce the particulate matter (PM) in the exhaustgas.

Preferably, the DOC is disposed upstream of the upstream of the SCRcatalyst, and more preferably upstream of the SCR reductant injector orNAC.

In a further embodiment, an oxidation catalyst for oxidizing nitrogenmonoxide in the exhaust gas to nitrogen dioxide can be located upstreamof a point of metering the nitrogenous reductant into the exhaust gas.In one embodiment, the oxidation catalyst is adapted to yield a gasstream entering the SCR zeolite catalyst having a ratio of NO to NO₂ offrom about 4:1 to about 1:3 by volume, e.g. at an exhaust gastemperature at oxidation catalyst inlet of 250° C. to 450° C. In anotherembodiment, the system further comprises a Closed Coupled Catalyst (CCC)upstream of the DOC.

The oxidation catalyst can include at least one platinum group metal (orsome combination of these), such as platinum, palladium, or rhodium,coated on a flow-through monolith substrate. Other metal catalysts thatcan be used in the DOC include aluminum, barium, cerium, alkali metals,alkaline-earth metals, rare-earth metals, or any combinations thereof.In one embodiment, the at least one platinum group metal is platinum,palladium or a combination of both platinum and palladium. The platinumgroup metal can be supported on a high surface area washcoat componentsuch as alumina, a zeolite such as an aluminosilicate zeolite, silica,non-zeolite silica alumina, ceria, zirconia, titania or a mixed orcomposite oxide containing both ceria and zirconia. In a preferredembodiment, the diesel oxidation catalyst composition contains about 10to 120 g/ft³ of a platinum group metal (e.g., platinum, palladium orrhodium) dispersed on a high surface area, refractory oxide support(e.g., γ-alumina).

In certain embodiments, one or more additional SCR catalyst componentscan be included in the system, preferably downstream of the SCRcatalytic filter, to further reduce the concentration of NO_(x) in theexhaust gas. For example, upon exiting the catalytic filter, the exhaustgas passes through a flow-through substrate coated with an SCR catalyst.Thus, the flow-through SCR catalyst is disposed downstream of the SCRcoated filter. The NO_(x) concentration of the exhaust gas is reduced asthe gas passes through the filter and then is further reduced as the gassequentially passes through the one or more SCR flow-through substrates.In another embodiment, the system further comprises an additionalreductant injector upstream of the SCR flow-through catalyst anddownstream of the dual function catalytic filter. In certainembodiments, the one or more downstream SCR flow-through catalyst areextruded articles.

The number of additional SCR catalyst flow-through components can be ofany practical number, such as 1, 2, 3, or 4. The downstream SCRcatalyst(s) may be the same or different from the SCR catalyst coated onthe SCR coated filter.

In certain embodiments, the system further comprises an ammonia slipcatalyst (ASC) disposed downstream of the dual function catalyticfilter, and in some embodiments, downstream of the flow-through SCRcomponents. The ASC serves to oxidize most, if not all, of the ammoniaprior to emitting the exhaust gas into the atmosphere or passing theexhaust gas through a recirculation loop prior to exhaust gasentering/re-entering the engine. Thus, the ASC reduces the concentrationof ammonia slip from the SCR reaction, the release of ammonia from thecatalyst surface during rapid temperature increases, or from the use ofa stoichiometric excess of reductant. Preferably, the ASC materialshould be selected to favor the oxidation of ammonia instead of theformation of NO_(x) or N₂O. Preferred catalyst materials includeplatinum, palladium, or a combination thereof, with platinum or aplatinum/palladium combination being preferred. Preferably, the catalystis disposed on a high surface area support, including but not limited toalumina. In certain embodiments, the ASC is applied to a substrate,preferably substrates that are designed to provide large contact surfacewith minimal backpressure, such as flow-through metallic or cordieritehoneycombs. For example, a preferred substrate has between about 25 andabout 300 cells per square inch (CPSI) to ensure low backpressure.Achieving low backpressure is particularly important to minimize theASC's effect on the low-pressure EGR performance. The ASC can be appliedto the substrate as a washcoat, preferably to achieve a loading of about0.3 to 2.3 g/in³. To provide further NO_(x) conversion, the front partof the substrate can be coated with just SCR coating, and the rearcoated with SCR and an NH₃ oxidation catalyst such as Pt or Pt/Pd on analumina support.

Methods of the present invention may also comprise one or more of thefollowing steps: (a) accumulating and/or combusting soot that is incontact with the inlet of a catalytic filter; (b) introducing anitrogenous reductant into the exhaust gas stream prior to contactingthe catalytic filter, preferably with no intervening catalytic stepsinvolving the treatment of NO_(x) and the reductant; (c) generating NH₃over a NO_(x) adsorber catalyst, and preferably using such NH₃ as areductant in a downstream SCR reaction; (d) contacting the exhaust gasstream with a DOC to oxidize hydrocarbon based soluble organic fraction(SOF) and/or carbon monoxide into CO₂, and/or oxidize NO into NO₂, whichin turn, may be used to oxidize particulate matter in particulatefilter; and/or reduce the particulate matter (PM) in the exhaust gas;(e) contacting the exhaust gas with one or more flow-through SCRcatalyst device(s) in the presence of a reductant to reduce the NOxconcentration in the exhaust gas; and (f) contacting the exhaust gaswith an AMOX catalyst, preferably downstream of the SCR catalyst tooxidize most, if not all, of the ammonia prior to emitting the exhaustgas into the atmosphere or passing the exhaust gas through arecirculation loop prior to exhaust gas entering/re-entering the engine.

EXAMPLE Example 1

About 100 grams of commercially available CHA aluminosilicate (SSZ-13)that was loaded with about 2.41 weight percent copper (based on thetotal weight of the aluminosilicate) was blended with about 100 gramscommercially available CHA silicoaluminophosphate (SAPO-34) that wasloaded with about 4.18 weight percent copper (based on the total weightof the silicoaluminophosphate) to form a blended powder.

The powder blend was exposed to a simulated diesel engine exhaust gasthat contained NOx and that was combined with ammonia. The catalyst'scapacity for NO_(x) conversion was determined at temperatures rangingfrom 200° C. to 550° C. For comparison, samples of similarly preparedCu:SSZ-13 and Cu:SAPO-34 were analyzed individually under similarconditions. FIG. 3 shows that all three samples had high freshperformance over the entire temperature window.

A powder blend prepared as above and separate samples of Cu:SSZ-13 andCu:SAPO-34 were aged at about 900° C. in the presence of about 4.5%water for four hours.

The aged samples were exposed to a simulated diesel engine exhaust gasthat contained NOx and that was combined with ammonia. The catalystsamples' capacity for NO_(x) conversion was determined at temperaturesranging from 200° C. to 550° C. FIG. 4 shows the hydrothermally aged NOxconversion of the three catalysts. Both the aluminosilicate andsilicoaluminophosphate catalysts alone show significant degradationafter this aging; however, a blend of the two components showssignificantly higher conversion over the entire temperature window.

What is claimed is:
 1. A catalyst composition comprising a blend of twomolecular sieve materials each having a CHA structure, wherein the firstmolecular sieve has a first silica-to-alumina ratio (SAR) of about 10-25and contains a first extra-framework metal, the second molecular sievehas second SAR of about 20-35 and contains a second extra-frameworkmetal, and wherein said first and second SAR are different and saidfirst and second extra-framework metals are independently selected fromthe group consisting of cesium, copper, nickel, zinc, iron, tin,tungsten, molybdenum, cobalt, bismuth, titanium, zirconium, antimony,manganese, chromium, vanadium, niobium, and combinations thereof, andwherein at least one of the first and second extra-framework metalscomprises manganese.
 2. The catalyst of claim 1, wherein the twomolecular sieve materials are aluminosilicates.
 3. The catalyst of claim1, wherein said first and second extra-framework metals areindependently selected from the group consisting of copper, iron, andmanganese, and wherein at least one of the first and secondextra-framework metals comprises manganese.
 4. The catalyst of claim 1,wherein at least one of the first molecular sieve and the secondmolecular sieve is an extrudate.
 5. The catalyst of claim 1, wherein thefirst extra-framework metal is present in an amount of about 1 to about5 wt % based on the total weight of the molecular sieve, and the secondextra-framework metal is present in amount sufficient to achieve aweight ratio of the first extra-framework metal and the secondextra-framework metal of about 0.4:1.0 to about 1.5:1.0.
 6. The catalystof claim 1, wherein the first molecular sieve and the second molecularsieve are present in a mole ratio of about 0.5:1:0 to about 1.5:1.0. 7.The catalyst of claim 1, wherein the catalyst composition is washcoatedon a wall-flow filter.
 8. The catalyst of claim 7, wherein the filtercontains inlet and outlet channels and the washcoat is on outletchannels.
 9. The catalyst of claim 1, wherein the catalyst compositionis washcoated on a flow-through honeycomb monolith.
 10. The catalyst ofclaim 9, wherein the flow-through honeycomb monolith further comprisesan oxidative underlayer.
 11. The catalyst of claim 10, wherein saidunderlayer comprises a platinum group metal.
 12. The catalyst of claim7, further comprising an ammonia oxidation catalyst disposed downstreamof the wall-flow filter.
 13. The catalyst of claim 9, further comprisingan ammonia oxidation catalyst disposed downstream of the flow-throughhoneycomb monolith.
 14. The catalyst of claim 7, further comprising aNO_(x) adsorber catalyst (NAC), a lean NO_(x) trap (LNT), or a NO_(x)storage/reduction catalyst (NSRC) located upstream of the wall-flowfilter.
 15. The catalyst of claim 7, further comprising an oxidationzone comprising a platinum group metal.
 16. The catalyst of claim 15,wherein the platinum group metal comprises Pd and/or Pt.
 17. A catalystcomposition comprising a blend of two molecular sieve materials having aCHA structure, wherein the first molecular sieve has a mean crystal sizeof about 0.01 to 1 μm and the second molecular sieve has a mean crystalsize of about 1-5 μm, and wherein the first molecular sieve contains afirst extra-framework metal, the second molecular sieve contains asecond extra-framework metal, and wherein said first and secondextra-framework metals are independently selected from the groupconsisting of cesium, copper, nickel, zinc, iron, tin, tungsten,molybdenum, cobalt, bismuth, titanium, zirconium, antimony, manganese,chromium, vanadium, niobium, and combinations thereof, and wherein atleast one of the first and second extra-framework metals comprisesmanganese.
 18. The catalyst of claim 17, wherein said first and secondextra-framework metals are independently selected from the groupconsisting of copper, iron, and manganese, and wherein at least one ofthe first and second extra-framework metals comprises manganese.
 19. Acatalyst composition comprising a blend of an aluminosilicate molecularsieve having a CHA framework and a silicoaluminophosphate molecularsieve having a CHA framework, wherein a. the aluminosilicate molecularsieve and the silicoaluminophosphate molecular sieve are present analuminosilicate:silicoaluminophosphate mole ratio of about 0.8:1.0 toabout 1.2:1.0; and b. said aluminosilicate molecular sieve contains afirst extra-framework metal, said silicoaluminophosphate molecular sievecontains a second extra-framework metal, wherein said first and secondextra-framework metals are independently selected from the groupconsisting of cesium, copper, nickel, zinc, iron, tin, tungsten,molybdenum, cobalt, bismuth, titanium, zirconium, antimony, manganese,chromium, vanadium, niobium, and combinations thereof, wherein saidfirst extra-framework metal is present in about 2 to about 4 weightpercent, based on the weight of the aluminosilicate, and wherein theweight ratio of said first extra-framework metal and said secondextra-framework metal is about 0.4:1.0 to about 1.5:1.0, and wherein atleast one of the first and second extra-framework metals comprisesmanganese.
 20. The catalyst of claim 19, wherein said first and secondextra-framework metals are independently selected from the groupconsisting of copper, iron, and manganese, and wherein at least one ofthe first and second extra-framework metals comprises manganese.